In previous studies it has been shown that murine monoclonal antibodies (MAb) of IgG2a isotype that bind to human tumor cells specifically inhibit growth of the tumor cells in nude mice. Recently, a Mab of IgG3 isotype has also been shown to be effective. Herlyn et al., (1980) Cancer Res. 40:71 7-721; Herlyn & Koprowski, (1982) Proc. Nato. Acad. Sci. U.S.A. 79:4761-4765. There was evidence suggesting that tumor growth inhibition by the MAb probably was mediated by macrophages since treatment of nude mice with silica abolished the tumoricidal effects of the MAb. Furthermore, antibody-dependent macrophage-mediated cytotoxicity (ADMC) assays with human tumor cells in cultyre resulted in specific lysis of these cells. Thioglycollate-elicited murine peritoneal macrophages were used in these assays. Human macrophages have also been shown to lyse tumor targets coated with MAb. Steplewski et al., (1983) Science 211:865-867. Macrophages, therefore, are strongly implicated as the effector cells mediating immunotherapeutic effects of, for example, MAb administered to gastrointestinal cancer patients. See. e.g., Koprowski in Proceedings of the IV Armand Hammer Cancer Synposium, pp. 17-38 (Boxx, Langman, Trowbridge & Dulbecco eds. 1984); Sears et al., (1984) J. Biol. Response Mod. 3:138-150.